A 20kW solar array sits in an odd but genuinely useful gap in the UK market. It’s too big for almost any domestic roof and too small to count as a proper commercial installation in the eyes of most installers who quote for factories and warehouses — but it’s exactly the right size for a farm shop, a rural workshop, a large B&B, a village hall with high daytime usage, or a small business unit with a three-phase supply and a hunger for cheaper daytime electricity. If you’ve been quoted anywhere between £18,000 and £26,000 and you’re wondering whether that’s fair, this is the sizing bracket you’re actually in — not quite residential, not quite commercial, and priced accordingly.
Why 20kW sits between two markets
Most UK homes top out at 4-6kW because roof space and the single-phase electrical supply limit them. Go much above that and you start running into two separate ceilings: physical roof area, and the capacity of a standard single-phase domestic connection (typically limited to around 3.68kW-16A per phase without District Network Operator approval, though inverters and export limitation devices can work around some of this). A 20kW system is usually only achievable on a large agricultural or commercial roof, a generously sized barn or workshop, or a property with a three-phase electricity supply already in place — which is precisely why it turns up so often on farm shops, rural workshops, equestrian centres, and small industrial units.
That puts 20kW squarely in “prosumer” territory: bigger than anything a typical domestic installer sizes for, smaller than the multi-hundred-kW arrays that commercialsolarpanelsinstallation.co.uk or solarpanelsforindustrialunits.co.uk deal with daily on distribution sheds and factory roofs. Get the sizing conversation right early, because the answer to “should I go bigger or smaller” hinges entirely on what your electrical supply and daytime usage profile actually look like.
The realistic cost band: £18,000-£26,000
For 2026, a well-specified 20kW system — decent-tier monocrystalline panels (often N-type TOPCon given where the market has moved), a suitably sized inverter (or string inverters split across roof faces), mounting, scaffolding/access, and installation labour — typically lands between £18,000 and £26,000 installed, before any battery storage. Where you fall in that range depends on:
- Roof type and pitch. A straightforward metal-clad agricultural roof with good south-facing pitch is materially cheaper to install on than a complex slate roof needing bespoke flashing.
- Scaffolding and access. Barns and workshops often need substantial scaffold or cherry-picker access, which adds real cost that a bungalow roof wouldn’t.
- Inverter configuration. A single large inverter is cheaper than splitting the array across multiple string inverters to handle east/west roof faces or partial shading — but shading and roof orientation sometimes force the split.
- Three-phase vs single-phase connection work. If you need a three-phase supply brought in or upgraded, budget separately for that — it’s a DNO (District Network Operator) job, not part of the solar quote, and can add several thousand pounds and weeks of lead time.
- Grid connection application (G99). At this size you’re virtually always filing a G99 application rather than the simpler G98 used for smaller domestic-scale systems, and DNOs can take weeks to respond, so build that into your timeline.
As always, get a written, itemised quote and use thecostofsolar.co.uk’s cost of solar panels UK guide as your baseline sense-check before signing anything — the same principles that apply to a 4kW quote (itemised panels, inverter, labour, scaffolding, and G98/G99 fees, not one lump sum) apply here too, just with bigger numbers attached.
The three-phase question, answered properly
This is the single most common confusion point at 20kW, so it’s worth being precise. Whether you need a three-phase supply depends on the inverter, not directly on the panels:
- Many string inverters up to around 15-17kW AC output can still run on a single-phase supply, provided your existing connection and the DNO’s export limits allow it.
- Above that, or if your property already has three-phase (common on farms, workshops, and older commercial premises), installers will usually spec a three-phase inverter, which is often more efficient overall, spreads load evenly, and avoids voltage rise issues that single-phase systems can hit at higher output.
- If you don’t currently have three-phase and your installer says you need it, get a firm cost and timeline from the DNO before you commit to the solar contract — this is the variable most likely to blow a “fixed price” quote off course, and it’s genuinely outside the installer’s control.
Any installer quoting a 20kW system without asking about your incoming supply phase and DNO connection agreement first hasn’t done the job properly. This is a question to ask upfront, not one to discover after the deposit’s paid.
Real-world use cases: farm shops and workshops
Farm shops and rural retail are a natural fit. These sites typically run daytime-heavy loads — refrigeration, chillers, lighting, card machines, catering equipment — that line up well with solar generation hours. A 20kW array on a farm shop roof, paired with the Improving Farm Productivity grant (around 25% of eligible cost for qualifying farm businesses in England — rates and schemes differ by devolved nation, so check current terms before assuming eligibility; this is not the old FETF 40% figure some outdated guides still quote), can materially cut the daytime electricity bill that refrigeration-heavy retail relies on. For farm-specific sizing and grant detail, solarpanelsforfarms.uk and solarpanelsforagriculture.co.uk both cover the agricultural angle in more depth than a generic commercial guide will.
Workshops and small industrial units — vehicle repair, joinery, engineering, light manufacturing — are the other classic 20kW candidate. These sites often run compressors, welding equipment, and machinery through the working day, which again matches solar output hours closely. If the unit sits within a wider industrial estate or distribution setting, it’s worth benchmarking against solarpanelsforindustrialunits.co.uk or, for costed comparisons across the commercial size range, commercialsolarcostuk.co.uk, which breaks down pricing per kWp across small, medium, and large commercial installs so you can see where 20kW sits relative to a 50kW or 100kW system.
Should you add a battery?
At 20kW, self-consumption is usually the whole point — you’re trying to offset high daytime grid electricity (around 25p/kWh at the Ofgem price cap level, though tariffs vary) rather than maximise export income, since Smart Export Guarantee rates vary by supplier and typically sit in the 12-20p/kWh range at the better end, well below what you’d pay to buy it back. If your business closes evenings and weekends and most of your load is daytime, you may not need a battery at all — you’re consuming what you generate in real time. If you run into the evening (a farm shop café, an equestrian centre with evening liveries, a workshop running two shifts), a battery makes more sense. Commercial-scale battery storage sized to match a 20kW array typically runs a similar cost logic to domestic batteries but scaled up — expect a meaningful additional outlay on top of the panel cost, and get a proper load-profile-based sizing recommendation rather than a round-number battery bolted on by default. batterystorageforbusiness.co.uk covers commercial battery sizing and cost in more depth if this is the direction you’re weighing.
Financing a 20kW system
At £18,000-£26,000, most businesses and farms aren’t paying cash outright. Commercial solar finance, asset finance, and Power Purchase Agreements (PPAs) all show up regularly at this size band — each with different implications for who owns the asset, who claims any capital allowances, and how the monthly cash flow compares to the electricity bill you’re offsetting. commercialsolarfinance.co.uk and solarassetfinance.co.uk both specialise in exactly this size and type of deal, and it’s worth getting a finance comparison alongside your installation quotes before deciding whether to buy, lease, or PPA the system — the “right” answer depends heavily on your business’s tax position and cash flow, not just the headline system price.
VAT and other cost considerations
The 0% VAT rate on residential solar and battery installations (in place across Great Britain until 31 March 2027, after which it’s scheduled to revert to 5%) applies to residential installations — a farm shop, workshop, or small business unit is a commercial installation and is normally standard-rated at 20% VAT, though VAT treatment can get genuinely complicated on mixed-use sites (a farmhouse plus a separate farm shop, for example), so get this confirmed by your accountant or installer rather than assuming either way. This VAT distinction is one of the most commonly misunderstood points when a domestic-scale system creeps into commercial-scale territory, and it can materially change your effective cost.
Getting quotes for a system this size
Because 20kW straddles residential and commercial installer specialisms, it’s worth getting quotes from both ends of the market rather than assuming one type of installer is automatically right. Regional installers with genuine larger-system experience — such as Ecoaim in Central Scotland, D&R Energy in Bristol on the commercial side, or EC Eco Energy in Essex — routinely handle installations in this bracket alongside smaller domestic jobs, which means they understand both the G99 grid application process and the practicalities of scaffolding a barn roof. Get at least three itemised quotes, ask each installer directly about phase requirements and DNO timelines, and don’t accept a single lump-sum price with no breakdown — at this cost, you’re entitled to see exactly what you’re paying for.
A 20kW system is a genuinely good investment for the right site — high daytime usage, decent roof or ground space, and either an existing three-phase supply or the appetite to arrange one. Get the electrical groundwork questions answered before you sign, get the finance options compared alongside the installation quotes, and treat the £18,000-£26,000 range as a starting sense-check rather than a fixed number until you’ve got itemised quotes in hand.